Add a new internal event for Duo Chat request

What does this MR do and why?

This MR adds a new internal event for tracking Duo Chat requests on SaaS / SM / Dedicated instances.

See https://gitlab.com/gitlab-org/gitlab/-/issues/452316+ for more information.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Confirmed that the event is tracked in Redis HLL counter (Service Ping):

bin/rails runner scripts/internal_events/monitor.rb request_duo_chat

Updated at 2024-04-03 05:59:15 UTC 
Monitored events: request_duo_chat

+----------------------------------------------------------------------------------------------------------------------------------------------------+
|                                                                  RELEVANT METRICS                                                                  |
+-------------------------------------------------------------------------+------------------+-----------------------+---------------+---------------+
| Key Path                                                                | Monitored Events | Instrumentation Class | Initial Value | Current Value |
+-------------------------------------------------------------------------+------------------+-----------------------+---------------+---------------+
| redis_hll_counters.count_distinct_user_id_from_request_duo_chat_response_monthly | request_duo_chat_response | RedisHLLMetric        | 1             | 1             |
| redis_hll_counters.count_distinct_user_id_from_request_duo_chat_response_weekly  | request_duo_chat_response | RedisHLLMetric        | 1             | 1             |
+-------------------------------------------------------------------------+------------------+-----------------------+---------------+---------------+

How to set up and validate locally

To set up Duo Chat feature, see https://docs.gitlab.com/ee/development/ai_features/#local-setup.